For Unity on 11.10+

Step one: Open up the Software Center.

To do that, open the Dash and search for Software - you'll see something like the screenshot below:

Open the Software Center by clicking on it. Wait for it to load.

Now, search for the software you want to install - in this example, "Chromium Web Browser".

Then, select the software and click the "More info" button.

From here you can view the version, the total amount of space it will take up once you've installed it, etc.

To install it, click the "Install" button on the right.

Once you've done that, you'll be asked to enter in your password - do that, then sit back and relax and Chromium will be installed.

You can change "chromium" to anything else that you need to get installed - Wunderlist, Geany, etc.

This is a comon problem with the software center. It seems not to like .deb packages it does not already have packages for and they chose to not install gdebi by default. You can get around this by installing gdebi yourself and using it, instead of software center, to install .deb packages.

For similar reasons you might choose to also install synaptic which is another package the chose to drop. Sorry Canonical, many of us are simply not Windows or Macintosh users and we don't like being treated like cattle.